Nintendo sold 113,000 3DS consoles in the UK during the first two days after the handheld's release. The UK tally accounted for more than a third of total sales in Europe.
The Nintendo 3DS has thus become Nintendo's fastest-selling handheld console, beating the DSi by over 20,000 units during the equivalent period.
Retailers …

No headaches...

...but when the best 3D effect on display is when you press the home button and the game fades back behind the menu screen, it kinda makes you think "What's the point?" My 9 year old son loves it though.

It's kids brains that are the trouble

Before the age of 4-6, they haven't finished wiring up the links between their eyeball muscles, focal changes, reflexes and signal analysis necessary to construct full depth perception out of two eyes. After they have that sorted, fake 3D like these won't unwire it.

Any problems adults have with it are therefore entirely their own. Lots of folks get motion sickness from playing first-person games on a normal screen, after all - or headaches from trying Magic Eye pics.

On top of which, everyone I know with one of these is an adult - too expensive for kids!

@Carol Orlowski

So you haven't been paying attention to the eye specialists who recommend that children SHOULD use the 3DS? Apparently it won't cause eye damage, but it may reveal pre-existing eye problems so they can be caught and treated early.